In this kind of burner, there is conventionally known one in which a combustion plate part is constituted by: a burner frame in the shape of a picture frame; a metal-fiber knit (i.e., a metal knit formed of a heat resistant metal fiber) which is disposed to cover an opening enclosed by the burner frame; and a distribution plate which has formed therein a multiplicity of distribution holes and which sandwiches the metal-fiber knit between the burner frame and the distribution plate. The air-fuel mixture is thus ejected through the distribution holes and the metal-fiber knit (see, for example, JP-A-2014-9839). The flame rod is disposed in a manner to lie opposite to a portion of the opening to be enclosed by the burner frame, i.e., to lie opposite to a portion of the metal-fiber knit.
When the flame rod is moved closer to the metal-fiber knit, frayed fibers of the metal-fiber knit may come into contact with the flame rod, thereby giving rise to wrong detection of the flame. Therefore, it has been made the practice to dispose the flame rod at a certain degree of distance to the metal-fiber knit. However, this practice has a problem in that, when the flame length has become considerably shorter in weak combustion, the flame ceases to contact the flame rod, thereby giving rise to a misjudgment that a misfiring has occurred.